LTM8029_1208 LINER [Linear Technology], LTM8029_1208 Datasheet - Page 12

• Manufacturer Part Number: LTM8029_1208
• Description: 36VIN 600mA Step-Down ?Module Converter with 5?A Quiescent Current
• Manufacturer: LINER [Linear Technology]

LTM8029
applicaTions inForMaTion
Frequency Selection
The LTM8029 uses a constant frequency PWM architec-
ture that can be programmed to switch from 200kHz to
2.2MHz by using a resistor tied from the RT pin to ground.
Table 2 provides a list of R
resultant frequencies.
Table 2. Frequency vs R
Operating Frequency Trade-offs
It is recommended that the user apply the optimal R
resistor value given in Table 1 for the input and output
operating condition. System level or other considerations,
however, may necessitate another operating frequency.
While the LTM8029 is flexible enough to accommodate
a wide range of operating frequencies, a haphazardly
chosen one may result in undesirable operation under
certain operating or fault conditions. A frequency that is
too high can reduce efficiency, generate excessive heat
or even damage the LTM8029 if the output is overloaded
or short-circuited. A frequency that is too low can result
in a final design that has too much output ripple or too
large of an output capacitor. In addition, as shown in the
Typical Performance Characteristics section, the operat-
ing frequency affects the amount of current that may be
delivered during a short-circuit condition.
12
FREQUENCY (MHz)
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
T
Value
T
resistor values and their
R
93.1
73.2
61.9
52.3
46.4
41.2
768
348
220
158
124
T
(k)
T
BIAS Pin Considerations
The BIAS pin is used to provide drive power for the in-
ternal power switching stage and operate other internal
circuitry. For proper operation, it must be powered by at
least 2.25V. If the output voltage is programmed to 2.25V
or higher, BIAS may be simply tied to V
than 2.25V, BIAS can be tied to V
source. If the BIAS pin voltage is too high, the efficiency
of the LTM8029 may suffer. The optimum BIAS voltage is
dependent upon many factors, such as load current, input
voltage, output voltage and switching frequency, but 4V to
5V works well in many applications. In all cases, ensure
that the maximum voltage at the BIAS pin is less than 25V
and that the sum of V
power is applied from a remote or noisy voltage source, it
may be necessary to apply a decoupling capacitor locally
to the pin.
Burst Mode Operation
To enhance efficiency at light loads, the LTM8029 auto-
matically switches to Burst Mode operation which keeps
the output capacitor charged to the proper voltage while
minimizing the input quiescent current. During Burst
Mode operation, the LTM8029 delivers single cycle bursts
of current to the output capacitor followed by sleep pe-
riods where the output power is delivered to the load by
the output capacitor. Since the LTM8029 is mostly shut
down between bursts, the effective switching frequency
will be lower than that programmed at the RT pin. For the
same reason, the output ripple will be different than when
the part is running at the full programmed frequency.
In addition, V
greatly reduced during the sleep time. As the load current
decreases towards a no load condition, the percentage of
time that the LTM8029 operates in sleep mode increases
and the average input current is greatly reduced, resulting
in higher efficiency.
IN
and BIAS quiescent currents are each
IN
and BIAS is less than 55V. If BIAS
IN
or some other voltage
OUT
. If V
OUT
is less
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